shut down the power, remove it and have a looksie inside and out. So long as the power is turned off (be sure) the most you have to lose is an already broken fan and possibly some scratched hands. Clean off any gunk and look for something obviously broken.
Culprits include bearings, motor brushes (unlikely used on a fan), or the switch. If this portion is beyond your skill, cruise youtube. If still beyond you, open the google pages and look for an electric motor or small appliance repair shop you can take it to.
If you are super handy you could also replace it with a similar sized product, like a bathroom exhaust fan.